1. Field of the Invention
The present invention relates to propellers, and more particularly to modular propellers used on boats that allow the individual blades to be replaced.
2. Description of Related Art
A plastic marine propeller and hub assembly is described by the present inventor, Brad Stahl, in U.S. Pat. No. 4,930,987, issued Jun. 5, 1990, herein “Stahl '987”. Three plug-in blade roots are slipped into an interlocking hub between front and rear end caps. All the parts are made of injection-molded high-strength fiber-reinforced plastic. The manufacturing costs of such propellers are said to be half the cost of conventional metal propellers. Typical applications are 16–18 inch diameter three blade propellers intended for 90–250 horsepower motors.
A double propeller is described in U.S. Pat. No. 5,423,701, issued Jun. 13, 1995, to Christian Rodskier, et al. The inner sleeves of the respective propeller hubs includes axial passages for venting the exhaust gases of the engine it is attached to. This arrangement is said to be novel for double propellers and increases the overall propeller efficiency.
Prior art commercial products similar to Stahl '987 taper the blade bases and corresponding center hub slots by 3–5 degrees. They are wider at the rear, and so the blades are replaced from the aft of the assembly. Such tapers were intended to help secure the blades in the hubs, but in practice the blades are loose in the hub slots. Replacing a broken blade in the field becomes much more difficult if it is jammed too tightly in the hub. The high taper angles also cause the blade to become loose while flexing under load in the completed assembly.